The present invention relates to a hydraulic pressure control system for an infinitely variable belt-drive automatic transmission for a vehicle.
A known control system for an infinitely variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulley in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The output of the engine is transmitted to the drive pulley through a clutch such as an electromagnetic clutch. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
FIG. 7 shows the relationship between the speed of a vehicle provided with the transmission and engine speed. In the graph, a line l is a maximum transmission ratio line and a line h is a minimum transmission ratio line. When an accelerator pedal of the vehicle is released to close the throttle valve of the engine to its idling position, the vehicle speed decreases along a transmission ratio increasing (downshifting) line m. When the vehicle speed decreases below a predetermined low speed (18 Km/h), e.g. v, the electromagnetic clutch is automatically disengaged so as to prevent stalling of the engine. During driving along the line m before disengagement of the clutch, the drive pulley and the engine are driven by the torque transmitted from the wheels of the vehicle through the driven pulley and the belt at a speed N.sub.P which is higher than the idling speed N.sub.ID of the engine. However, when the clutch is disengaged, the engine speed drops to the idling speed N.sub.ID. The speed N.sub.P is, for example 1500 rpm and the idling speed is 800 rpm. Therefore, there is a considerable speed difference N.
Under this condition, when the accelerator pedal is depressed in order to accelerate the vehicle and the engine speed slightly increases, the clutch is automatically engaged. However, since the difference between the engine speed and the drive pulley speed is large, a great shock occurs upon engagement of the clutch. Further, since the transmission ratio is large at that time, there is a large engine braking operation effect on the vehicle, which produces uncomfortable shock.